Hydraulic fracturing is a method of using pump rate and hydraulic pressure to fracture or crack a subterranean formation in a process for improving the recovery of hydrocarbons from the formation. Once the crack or cracks are made, high permeability proppant, relative to the formation permeability, is pumped into the fracture to prop open the crack. When the applied pump rates and pressures are reduced or removed from the formation, the crack or fracture cannot close or heal completely because the high permeability proppant keeps the crack open. The propped crack or fracture provides a high permeability path connecting the producing wellbore to a larger formation area to enhance the production of hydrocarbons. The development of suitable fracturing fluids is a complex art because the fluids must simultaneously meet a number of conditions. For example, they must be stable at high temperatures and/or high pump rates and shear rates that can cause the fluids to degrade and prematurely settle out the proppant before the fracturing operation is complete. Various fluids have been developed, but most commercially used fracturing fluids are aqueous based liquids that have either been gelled or foamed. When the fluids are gelled, typically a polymeric gelling agent, such as a solvatable polysaccharide, for example guar and derivatized guar polysaccharides, is used. The thickened or gelled fluid helps keep the proppants within the fluid. Gelling can be accomplished or improved by the use of crosslinking agents or cross-linkers that promote crosslinking of the polymers together, thereby increasing the viscosity of the fluid. One of the more common crosslinked polymeric fluids is borate crosslinked guar.
The recovery of fracturing fluids may be accomplished by reducing the viscosity of the fluid to a low value so that it may flow naturally from the formation under the influence of formation fluids. Crosslinked gels generally require viscosity breakers to be injected to reduce the viscosity or “break” the gel. Enzymes, oxidizers, and acids are known polymer viscosity breakers.
While polymers have been used in the past as gelling agents in fracturing fluids to carry or suspend solid particles as noted, such polymers require separate breaker compositions to be injected to reduce the viscosity. Further, such polymers tend to leave a coating on the proppant and a filter cake of dehydrated polymer on the fracture face even after the gelled fluid is broken. The coating and/or the filter cake may interfere with the functioning of the proppant. Studies have also shown that “fish-eyes” and/or “micro-gels” present in some polymer gelled carrier fluids will plug pore throats, leading to impaired leakoff and causing formation damage.
Aqueous drilling and treating fluids may be gelled or have their viscosity increased by the use of non-polymeric viscoelastic surfactants (VES). These VES materials are advantageous over the use of polymer gelling agents in that they do not leave a filter cake on the formation face, do not coat the proppant or create micro-gels or “fish-eyes”, and have reduced potential for damaging the formation relative to polymers. However, many aqueous base fluids, e.g. a brine, prepared in field operations become contaminated with metal ions and particles due to metal corrosion.
These metal ions may be present because of impurities in the salt products used to prepare aqueous base fluids and/or aqueous treating fluids or from metal surfaces of mixing and handling equipment. It is well know that saline water may corrode, leech and/or dissolve metal ions from metal surfaces and metal-scale particles. In one non-limiting example, the metal ions may be from the source water of the aqueous base fluid or may be due to the corrosion of metal surfaces from brine mixing systems and/or stocking tanks. In another non-limiting example, the source of the metal ions may be from the metal surfaces and/or scale in piping, valves, and pumps that may be used to transfer the brines. In still another non-limiting example, the metal ions may be from the equipment used to mix an aqueous treating fluid. In yet another non-limiting example, the metal ions may be from the VES product or other additive, or from the wellbore tubing or casing utilized during the VES treatment.
The contamination by metal ions may occur from many different types of sources, but regardless of the source, the metal ions alter the VES into an undesirable form. The presence of the metal ions may degrade, redox, or alter the VES gel viscosity haphazardly and uncontrollably, in some cases even upon fluid heat-up. In such cases the redox reactions by metal ions is certainly unwanted.
It would be desirable if the metal ions within the aqueous treating fluid could be complexed and thereby prevent the degradation and/or redox of the VES by the metal ions.